Exemplary embodiments of the invention relate to a fuel cell system having at least one fuel cell.
German patent document DE 101 15 336 A1 discloses a fuel cell system having a recirculation device for recirculating the anode exhaust gas into the anode input. In these types of systems nitrogen and water become concentrated in the recirculated anode exhaust gas over time. For this reason, it is known from the general prior art and also described in the above-cited publication that valve units are situated in the area of the recirculation line which are occasionally opened in order to appropriately blow off the nitrogen from the region of the recirculation line and the region of the anode chamber. According to DE 101 15 336 A1, the “disposal” of this exhaust gas from the area of the anode loop may take place in various regions, which typically each have a catalytic surface or are in connection with a further component having such a catalytic surface. This design is thus common, since together with the nitrogen, a small quantity of hydrogen will also always be present in the discharged gas, which may be made harmless in this manner. DE 101 15 336 A1 discloses a water separator in the region of the recirculation line to allow discharge the product water of the fuel cell occurring in the area of the anode exhaust gas.
A fuel cell system having a recirculation device is likewise known from International Application WO 2008/052578 A1, which is referred to therein as a fuel circuit. The special feature of this design is that the functionality of the water separator having a discharge valve for discharging the water and the functionality of the relief valve for blowing off the nitrogen-containing gas are combined. According to the design, a water separator having a corresponding valve unit is provided. Whenever an appropriately large quantity of water has accumulated, it is discharged from the water separator via the valve unit. After the water has been discharged, gas also exits from the recirculation device via the valve unit of the water separator before the valve unit is closed once again. The functionality, which in the above-cited publication is distributed over two independent components, is thus integrated into a single component.
The problem with the simultaneous discharge of water and gas arises when the discharge occurs in the cathode chamber of the fuel cell, which is indicated in the cited WO publication as optional. The catalyst, which is present there anyway, may be utilized to avoid possible hydrogen emissions. On the other hand, however, the water, which is likewise discharged from the region of the recirculation device, may result in impairment of the functionality of the fuel cells. Tests have shown that this introduction of water may impair the efficiency of individual fuel cells, thus adversely affecting the overall performance of the fuel cell.
Exemplary embodiments of the present invention are directed to an improved fuel cell system that avoids, by a simple system design, these functional impairments minimizes the amount of catalyst material required.
According to the invention a fuel cell system having a recirculation device has a design that is comparable to the prior art, and via a discharge line conducts liquid and/or gas from the region of the recirculation device into the region of a supply air line or a supply air flow to the cathode chamber of the fuel cell. According to the invention, a water separator, which is connected to the discharge line and through which at least a portion of the supply air flow passes, is situated between the air conveying device and the cathode chamber. This water separator is used to separate the water conveyed from the mixture through the discharge line and otherwise discharge it from the fuel cell system, so that liquid water is not able to pass into the region of the cathode chamber. On the other hand, the aim is for the gaseous portion, which flows through the discharge line from the region of the water separator and which typically contains a residual amount of hydrogen, to be securely and reliably conducted into the region of the cathode chamber in order to prevent hydrogen emissions from the fuel cell system to the environment.
In another very advantageous embodiment of the fuel cell system according to the invention, a partial flow of the supply air flow passes through the water separator. According to one very advantageous refinement, the partial air flow constitutes up to 25 percent, preferably approximately 10 percent, of the volume flow of the supply air flow. As the result of only a partial air flow of the supply air flow being conducted through the water separator, a comparatively low flow velocity of the partial air flow and of the gas mixture originating from the discharge line may be achieved in the region of the liquid separator, even with a small installation size. Such a low flow velocity is a prerequisite for reliable separation of liquid from the liquid-gas mixture. As a result, all of the gas passes into the region of the cathode chamber, where the hydrogen present in the gas completely reacts on the catalytic material of the cathode chamber, thus preventing hydrogen emissions to the environment.
Such a water separator through which only a partial air flow passes thus allows a small installation size with very good separation, so that the introduction of liquid water into the cathode chamber may be largely avoided, thus enabling higher efficiency and a longer service life of the fuel cell.
In another very advantageous embodiment of the fuel cell system according to the invention, the partial air flow from the water separator flows upwardly in the direction of the force of gravity during proper operation, and upstream from the cathode chamber is recombined with the remainder of the supply air flow from which it has branched off. This arrangement in which the partial air flow passes vertically upwardly through, and in particular downstream from, the water separator for at least a segment further improves the separation of the water.
In another very advantageous embodiment of the fuel cell system according to the invention, it may also be provided that a catalytic material that facilitates the reaction of hydrogen with oxygen to form water is situated in the region of the water separator. Such a catalytic material may be situated in the region of the water separator, for example in the form of a coating or the like. The catalytic material may ensure that hydrogen, which passes via the discharge line into the region of the water separator, is reacted with oxygen from the partial air flow of the supply air flow to form water. A suitable mixture may be achieved by an appropriate selection of the magnitude of the supply air flow. Thus, the conversion of the hydrogen is already achieved at least partially outside the cathode chamber, and the resulting water is appropriately discharged via the water separator. When the water separator is dry and the valve unit is open in a water line for discharging the water, such a catalytic material in the region of the water separator may also thus ensure that little or no hydrogen passes to the environment, since, due to the catalytic material in the region of the water separator, the hydrogen is reacted, at least partially, with the atmospheric oxygen to form water.
In another very advantageous embodiment of the fuel cell system according to the invention, the water separator is actively or passively heatable. The water separator may be actively heated, for example, via an electric heater or the like, since with regard to a cold start of the fuel cell system at temperatures below the freezing point, the water separator may be critical with respect to freezing water. This water could be thawed via such active heating. In principle, of course, it would also be conceivable to carry out the heating in some other way, for example passively, in that the water separator is in thermal contact with a heated component on the part of the water separator, and/or is in thermal contact with a component that is heated by the start-up operation of the fuel cell, for example in contact with the cooling water.